Openness as a resource: Accessing new quantum states with dissipation

The two-point measurement (TPM) scheme is one of the standard approaches to define work in non-equilibirum quantum thermodynamics. The energy of a closed system is measured projectively at the beginning and at the end of the protocol. The work for a single run is then given by the energy difference. It is well known that a projective TPM scheme cannot be replaced by a single work measurement observable that reproduces both the TPM statistics for diagonal input states, and the correct average work. However, projective measurements are an idealization and real experiments suffer from noise. Therefore, we extend the scenario to unsharp energy measurements in the TPM scheme and show that the no-go theorem does not apply if the (unsharp) energies at the beginning and at the end of the protocol can be measured jointly for any intermediate unitary evolution. In such a case a work observable can exist.

Environmental coupling typically drives a quantum system to a unique steady state with little quantum coherence, which is a major obstacle for quantum information processing. I will present a simple experimental setting of an array of two-level systems with localised environmental noise that has multiple highly coherent steady states, including maximally-entangled states of nonlocal (Bell) pairs. Such states originate from a hidden symmetry that conserves these pairs over long distances, leading to controllable long-range entanglement. I will discuss how to selectively prepare and observe these states in present-day atomic/photonic setups.

We study the escape rate of a particle in a metastable potential at zero temperature in the presence of a dissipative bath coupled to the momentum of the particle and find that this rate is exponentially enhanced. In particular, the influence of momentum dissipation depends on the form of the barrier that the particle is tunneling through. We investigate also the influence of dissipative baths coupled to the position, and to the momentum of the particle, respectively. In this case the rate exhibits a nonmonotonic behavior as a function of the dissipative coupling strengths [1]. Our theoretical findings can be tested in superconducting quantum circuits in which dissipative position and momentum interactions translate to dissipative phase or charge couplings. In particular, momentum/charge dissipation can be implemented using capacitors and resistors [2]. We propose a circuit based on a current biased Josephson junction and two resistors coupled to the charge and phase, respectively. We insert realistic circuit parameters and find that the dissipative coupling to the charge can lead to a significant enhancement of the tunneling rate of the phase even in presence of a second environment favoring localization.

Real quantum systems are seldom isolated. The natural question to ask is if the coupling to the environment will trigger new phenomena, and, if so, at which energy or time scale. This question is not only relevant in the realm of quantum simulation or computing, but also in the solid state. In this talk, we will consider an $S=1/2$ antiferromagnetic quantum Heisenberg chain where each site is coupled to an independent bosonic bath with ohmic dissipation [1]. The coupling to the bath preserves the global SO(3) spin symmetry. Using our recently-developed wormhole quantum Monte Carlo method [2], we show that any finite coupling to the bath suffices to stabilize long-range antiferromagnetic order. This is in stark contrast to the isolated Heisenberg chain where spontaneous breaking of the SO(3) symmetry is forbidden by the Mermin-Wagner theorem. A linear spin-wave theory analysis confirms that the memory of the bath and the concomitant retarded interaction stabilize the order. For the Heisenberg chain, the ohmic bath is a marginal perturbation so that exponentially large systems sizes are required to observe long-range order at small couplings. Below this length scale, our numerics is dominated by a crossover regime where spin correlations show different power-law behaviors in space and time. Our recently-developed wormhole quantum Monte Carlo method will enable many future studies of dissipative interacting quantum systems in thermal equilibrium. [1] M. Weber, D. J. Luitz, F. F. Assaad, arXiv:2112.02124 [2] M. Weber, arXiv:2108.01131

Quantum Computing in the Context of Artificial Intelligence